Photovoltaic cells such as CdS/Cu.sub.x S solar cells are capable of converting solar radiation into usable electrical energy. The energy conversion occurs as a result of what is well known in the solar field as the photovoltaic effect. Solar radiation impinging on a solar cell is absorbed by a semiconductor layer which generates electrons and holes. The electrons and holes are separated by a built-in electric field, for example, a rectifying junction such as a PN junction in the solar cell. The electrons flow towards the N-type material and the holes flow towards the P-type semiconductor material. The separation of the electrons and holes across the rectifying junction results in the generation of an electric current known as the photocurrent and a voltage known as the photovoltage. The performance of CdS/Cu.sub.x S solar cells is in part dependent on the sharpness of transition from the layer of CdS to the layer of Cu.sub.x S. The optimum composition of the Cu.sub.x S layer is in the range where x varies from 1.95 to 2.0, preferably higher than 1.995. Many heat treatments in H.sub.2 are needed to achieve optimum device efficiencies. Thus, it would be highly desirable to find a method of insuring increasing x to improve the efficiency of the solar cell and shorten the processing time and steps.